1. Field of the Invention
This invention relates to a device for cooling the last rows of rotating blades and stationary vanes of a steam turbine and, more specifically, to a device that combines water and steam to deliver a dual-fluid having a fine droplet size to the last rows of rotating blades and stationary vanes of a steam turbine.
2. Background Information
Steam turbines are well known in the prior art. Such turbines include a casing which houses rows of stationary vanes and rotating blades. Compressed working steam expands while passing through the vanes and blades, causing the blades to rotate. The blades cause a shaft, which is coupled to a generator, to rotate, thus allowing power to be generated.
With advances in steam turbine design, turbine blades now have large enough diameters and rotate at a sufficient speed that windage heating during a shut down of the turbine creates temperatures which approach the operating limits of the blades and vanes. Particularly during shut down of the turbine, the normal flow of working steam is effectively terminated by closure of the valves admitting steam into the turbine. Any fluid, e.g., residual steam in the turbine, tends to remain within the turbine and/or the exhaust region. That is, any fluid within the turbine and/or the exhaust region does not move significantly upstream or downstream from the turbine and/or the exhaust region. These conditions are characterized by a strong recirculation and a backflow from the exhaust region through the last stage of the turbine. As the blades rotate at high speeds, e.g., near normal operating r.p.m., the recirculating fluid which is trapped in the exhaust region is heated due to friction. Heat from the fluid is transferred to the blades and vanes. Such heating can cause the blade and vane temperature to rise to above 600.degree. F. Allowing the blades to reach these temperatures reduces the margin between material strength (which is temperature dependent) and operating stresses (which are speed dependent). During a start-up of the turbine, the flow through the turbine is reduced to about 3 to 5% of normal flow. Under these conditions, windage heating and recirculation occurs, but is not as severe.
To maintain the margin between strength and stress limits of the blades, cooling devices are used to reduce the temperature within the turbine and/or the exhaust region during start-up and shutdown sequences. Prior art cooling devices for steam turbines include mechanisms which inject water droplets into the flow path. These water droplets typically have a Sauter mean droplet size of 300-400 microns. One disadvantage of such devices is that the larger droplets require a disproportionately long time to complete the evaporation process, thus reducing cooling effectiveness. A second disadvantage is that the larger water droplets cause erosion damage over time as the droplets impact on the rotating blades.
Therefore, there is a need for a cooling device for the exhaust region of a steam turbine that produces a very fine cooling spray that will provide effective evaporative cooling but which will not cause erosion damage to the rotating blades.
There is a further need for a cooling device that is adaptable for use with existing steam turbines.